One violation of the biblical injunction not to "place a stumbling block in the path of the blind" is aiding and abetting an illegal or unethical transaction.

Blocking the Path of the Blind

There are acts in commerce and business that, although there is neither theft nor fraud, either of price or quality, nevertheless present moral problems.
It may well be that there is no financial damage, but the action may not be permissible. In the main they are directed at the effect on the spiritual and moral well being of one the parties.

The Jewish frame of reference for dealing with these issues is the biblical injunction “You shall not put a stumbling block in the path of the blind” (Leviticus 19:14). In interpreting this verse the Rabbis did not see it as referring to the prevention of placing physical objects in the path of a blind man. That was understood as part of the natural morality common to all mankind. The Torah comes to add dimensions, divinely revealed, to this natural morality, and so lifnei ivver has to be understood as having wider ramifications. Indeed the Sages understood lifnei ivver as referring to the giving of unwise business advice to someone or the supply, through perfectly legal transactions, of goods that are to the buyer’s moral detriment.

The emphasis is on the moral detriment of the recipient. It is true that one may not sell or give another goods that are physically harmful to him, nor may one, it may be assumed, advertise or encourage him to buy them from somebody else. This would include the production and sale of cigarettes or drugs. However it is not necessary to apply lifnei ivver here. Halakhically, a man may not cause damage to his own or another’s body or to his property, either through the body or through the misuse of property. Such injunction comes under the category of nezek, damages, and the injured party has recourse to the rabbinic court. Lifnei ivver, it would seem, refers to a different category of actions primarily affecting the moral well being of both parties.

Maimonides codifies lifnei ivver as follows:

“Everyone who causes the blind to stumble in anything [in which he is not knowledgeable] and knowingly tenders him advice that is to his detriment, or those who strengthen the hands of evildoers who are [figuratively] blind and who because of their lusts do not see the true way [of the Torah and the Divine Commandments] transgresses a negative commandment, even as it is written: ‘You shall not put a stumbling block in the path of the blind.’” (Maimonides, Mishneh Torah, Laws of Manslaughter 12:14)

Broadly speaking, therefore, lifnei ivver in the marketplace may be considered in two different forms: assisting a person to perform actions that are detrimental to himself -- mesaya li-dvar aveirah; and the giving of bad advice, as distinct from incorrect advice. In both cases, the “blind person” is considered to be the one who is blind or willfully blinds himself to the consequences of what he is doing. Most of the rabbinic responsa and legal codes refer to the first form of lifnei ivver.

Dr. Meir Tamari, former chief economist in the office of the Governor of the Bank of Israel, is director of the Center for Business Ethics at the Jerusalem College of Technology. His books include Al Chet: Sins in the Marketplace (Jason Aronson) and Jewish Values in Our Open Society: A Weekly Torah Commentary (Jason Aronson).